A customized in-ear hearing device includes a light source, photodetector, light emission window, and light transmission window. The light source and the photodetector together function as a physiological sensor by emitting light from the light source through the light emission to the skin of a user's ear canal, and detecting at the photodetector light reflected by the skin that passes through the light detection window. The light emission window and light detection window are located at a customized portion of the sidewall of the device that contacts the skin in the ear canal.